In reality its only 14-24 months extra to get a separate degree.

Option 1:
You get your 4 year degree in aviation you get your CFI in 4 years then spend another 6 months - 1 year building hours. (reduced hours requirements for some of the 4 year programs)

Option 2:
You spend 4 years getting your degree in something besides aviation. You then spend 8 months - 1 year getting your CFI and another year to 18 months building hours.